This invention relates to circuit protection apparatus and, more particularly, to telecom protection apparatus for the protection of telecom (telecommunications) equipment against diverse forms of abnormal, potentially dangerous line events such as lightning surge and power cross, protection against which dictates the application of contradictory design strategies.
Most telecom equipment that interfaces directly with the outside lines of service providers is required by regulatory agencies to employ protective measures against catastrophic failure or fire in the event that it is exposed to line anomalies, such as lightning surges or inadvertent line contacts with high voltage power feeds (power cross). A typical regulatory approach for equipment qualification (such as is put forth in Telcordia GR-1089) is to subject a device to a series of well-defined overload simulations representative of both types of anomalies. These specified tests are divided into two levels of severity; those that the protected equipment must survive without functional impairment and those under which it may fail so long as it presents no fire or safety hazard.
Designing protection circuits to handle both lightning surge and power cross faults with this defined dividing line between survival and controlled failure requires a balance between two, conflicting approaches. Lightning transients are high-energy events of very short duration that can induce very high current spikes at a level of several thousand volts. Power cross events usually involve contact with AC feeds carrying 600 volts or less and can exist for protracted periods. One typical solution for protecting against these diverse types of faults involves the insertion of a “transient tolerant” fuse or PTC (Positive Temperature Coefficient) device in series with one or both of a service line's feed wires (commonly designated as “Tip” & “Ring”) along with a thyristor or other voltage clamping device placed between the equipment side of each fuse/PTC and a grounding point, thereby providing a fast response means of clamping excessive voltage at a safe level. Many circuit permutations exist, but the findamental requirement for one or more transient tolerant fuses/PTCs exists in each protection scheme.
To be suitable for this application, a fuse/PTC device must be able to conduct brief, high peak currents associated with lightning surges to an associated voltage clamping device without opening. This implies that the fusing element should have a high enough I2t (energy let-through) to preclude its opening when exposed to “acceptable” surge levels resulting in unwanted equipment failure. However, the fuse must also be responsive enough to open within a relatively short, specified time when exposed to an agency defined, low-level, overcurrent fault. Meeting this second requirement restricts the allowable current rating of the fuse, lowering its I2t which is roughly proportional to fuse element mass. These conflicting requirements severely constrain the design parameters of the fuse.